The present disclosure relates to positively chargeable monolayer electrophotographic photosensitive members which include a photosensitive layer containing a hole transport material and an electron transport material each of which has a particular structure. The present disclosure also relates to image forming apparatuses including the positively chargeable monolayer electrophotographic photosensitive member as an image bearing member.
A type of electrophotographic image forming apparatus includes an electrophotographic photosensitive member. The electrophotographic photosensitive member is either an inorganic photosensitive member or an organic photosensitive member. The inorganic photosensitive member includes a photosensitive layer formed of an inorganic material such as selenium or amorphous silicon. The organic photosensitive member includes a photosensitive layer mainly formed of organic materials such as a binder resin, a charge generating material, and a charge transport material. The organic photosensitive member is more easily produced than the inorganic photosensitive member. The organic photosensitive member provides high design flexibility because there is a wide choice of materials for the photosensitive layer. Therefore, of these photosensitive members, the organic photosensitive member is more widely employed.
An example of the organic photosensitive member is a monolayer organic photosensitive member including a photosensitive layer which contains a charge generating material and a charge transport material in the same layer. It is known that the monolayer organic photosensitive member has a simpler structure and is more easily produced than a multilayer organic photosensitive member. It is also known that the occurrence of defective film can be reduced or prevented. The multilayer organic photosensitive member has a structure including a charge generating layer which contains a charge generating material and a charge transport layer which contains a charge transport material, the two layers being stacked together on a conductive substrate.
Such an electrophotographic photosensitive member is used to perform an image forming process including the following steps 1)-5):
1) charging a surface of the electrophotographic photosensitive member;
2) exposing the charged surface of the electrophotographic photosensitive member to light to form an electrostatic latent image;
3) developing the electrostatic latent image using toner in the presence of an applied development bias voltage;
4) transferring the formed toner image to a transfer member; and
5) fixing the toner image transferred to the transfer member by heating.
The electrophotographic photosensitive member is rotated for use during such an image forming process. Therefore, a phenomenon occurs that the potential (light potential) of a portion which has been exposed during image formation remains, and therefore, even after the charging step in the next turn of the photosensitive member, a desired charge potential (dark potential) cannot be obtained at the portion which has been exposed in the previous turn. This phenomenon is called “transfer memory.” Portions with and without transfer memory have different image densities, and therefore, it is difficult to obtain a satisfactory image.
There are two different types of monolayer electrophotographic photosensitive members, i.e., positively chargeable type and negatively chargeable type. There are also two different types of techniques of charging the electrophotographic photosensitive member, i.e., contact charging and non-contact charging. In the positively chargeable monolayer electrophotographic photosensitive member, when the surface of the electrophotographic photosensitive member is charged, substantially no oxidizing gas such as ozone occurs, which adversely affects the life of the electrophotographic photosensitive member or the office environment. Therefore, the positively chargeable monolayer electrophotographic photosensitive member is preferably used. The positively chargeable monolayer electrophotographic photosensitive member is more preferably used in combination with a contact-charging charger. However, when the contact-charging charger and the positively chargeable monolayer electrophotographic photosensitive member are used in combination, transfer memory is particularly likely to occur.
Under the above circumstances, there is a demand for a positively chargeable monolayer electrophotographic photosensitive member which can reduce or prevent the occurrence of transfer memory during image formation. The use of a charge transport material having excellent charge transport performance is effective in reducing or preventing the occurrence of transfer memory. As charge transport materials having excellent charge transport performance, a variety of triarylamine derivatives, which are hole transport materials, have been proposed. Specific examples of a suitable triarylamine derivative as a hole transport material include the following compounds (HTM-A and HTM-B):
